In these machines, the tracks constitute a pair of steering tracks with large surfaces for bearing on the ground, the arrangement of which on the ground is changed by the operation of toothed drive wheels which engage the tracks, whilst the weight of the vehicle, which is often considerable, is supported by lower support rollers which are free to rotate on upper sides or running surfaces of the portions of the tracks which are bearing on the ground. 
Further, upper rollers, on the other hand, have the function of guiding the return portions of the tracks which are not bearing on the ground. 
When the tracked vehicle is in motion, the rollers rotate with the tracks by friction, with a considerable component of sliding and impact friction due to the discontinuity of the links and to the different relative attitudes of the portions of track formed by the links, which are not generally perfectly aligned.  
This leads to considerable wear of the tracks and of the support and guide rollers, further aggravated by the environment in which these machines have to operate which causes loose and highly abrasive earthy material to be carried along and interposed between the tracks and the rollers. 
The structure of the shaft/roller units for tracked vehicles is therefore conditional upon the need to satisfy and to reconcile to some extent different requirements imposed by the operating conditions:                 the rollers should be particularly wear-resistant and easily replaceable,         the axial positioning of the rollers on the shaft should be ensured by restraints which can withstand the considerable axial thrusts which develop not only when the tracked vehicle has to change its direction of travel or to rotate about itself, but also when the machine discharges onto the tracks, stresses oriented transversely relative thereto, in working conditions;         with regard to the considerable loads to be withstood, the kinematic coupling between the movable roller and the fixed shaft should provide adequately lubricated bearings (bushings) protected by seals (also replaceable) which prevent leakage of lubricant, on the one hand, and infiltration of abrasive earthy substances  into the coupling, on the other hand;         in order to reduce the contact area and the sliding speed of the seals, they should have as small a diameter as the size of the shaft allows, particularly in shaft/roller units used on tractors for which the speed of movement, and hence the speed of the rollers, is quite fast, with working cycles of considerable, almost continuous, duration during movement.         
Various arrangements have been proposed to satisfy and reconcile these requirements and all are based on a structural concept which provides for replaceability solely of the rollers or even solely of their peripheral portions. 
The cost of this is structural complications which greatly increase the cost of the unit, make replacement operations onerous, and in any case are at least partially detrimental with regard to other requirements. 
These problems are completely overcome by the shaft/roller unit for tracked vehicles of the present invention which, since it is based on the criterion of providing for replaceability and exchange of the entire shaft/roller unit, achieves considerable structural simplifications and substantial reductions in cost and ensures a particularly long useful life of the unit, much longer than that of most units of the prior art.  